Fatigue crack growth was investigated in thin sheets of electrodeposited nickel–1.6% cobalt (Ni-Co) and rolled Inconel 718. Digital image correlation (DIC) was used to determine the crack opening displacement profiles, and the entire crack displacement fields. These parameters were then used to determine the magnitude of the crack tip stress intensity factor, and virtual extensometers were used to establish precisely the crack opening stress levels. The use of DIC enhanced the processing of experimental data and images, and gave detailed view of the crack tip stress intensity evolution with increasing crack length, transcending the limitations of previous measurement techniques.
The Ni-Co alloy exhibits planar slip, has a nanocrystalline grain structure and high density of nanotwins as shown with extensive transmission electron microscopy. The crack growth rate exponent is less than 1.5 and the growth rates are finite stress intensity ranges as high as 100. We relate the fatigue crack growth resistance in the Ni-Co to slip-twin interactions, and the difficulty of slip to penetrate grain boundaries and annealing twins.